1. Field of the Invention
The present invention relates to a method of dividing a semiconductor wafer formed with a plurality of streets to partition a plurality of device forming areas from each other, and more particularly to a dividing method for a wafer strip.
2. Description of the Related Art
A semiconductor wafer (which will be hereinafter referred to simply as a wafer) is formed with a plurality of crossing streets (division lines) to partition a plurality of device forming areas in which a plurality of devices are respectively formed. After forming a device in each device forming area of the wafer, the wafer is polished, for example. Thereafter, the wafer is cut along the streets to obtain the individual devices (semiconductor chips). Examples of a wafer cutting method include a method using a cutting blade and a method of applying a laser beam along the streets to form slits on the wafer by utilizing a thermal evaporation phenomenon called ablation.
Known as another example of the wafer cutting method is a method including the steps of first forming a modified layer as a weak area inside the wafer and next applying an external force to the wafer to thereby divide the wafer along the modified layer (see Japanese Patent Laid-open No. 2005-95952, for example). According to this method, the focal point of a transmissive laser beam is set inside the wafer, and the laser beam is applied along each street of the wafer to thereby form the modified layer inside the wafer along each street. Thereafter, an external force is applied to the wafer to thereby break the wafer along the modified layer as an easily breakable area, thus dividing the wafer along each street to obtain a plurality of individual devices.
There is a case that the wafer is divided into a plurality of wafer strips along some of the streets before it is divided into the individual devices. In this case, each wafer strip is divided along the other streets to obtain a plurality of devices. FIG. 3A shows a wafer 1 to be divided into such a plurality of wafer strips, and FIG. 3B shows one of the wafer strips obtained by dividing the wafer 1 shown in FIG. 3A. As shown in FIG. 3A, the wafer 1 has a front side 1a, which is formed with a plurality of first streets 2 and a plurality of second streets 3 intersecting the first streets 2 at right angles to thereby partition a plurality of rectangular device forming areas 4 from each other. The wafer 1 is first divided along the first streets 2 to obtain the plural wafer strips. Thereafter, a laser beam is applied to each wafer strip 1 along the second streets 3, thereby forming a modified layer 5 (see FIGS. 4A and 4B) inside each wafer strip 1 along each second street 3. Thereafter, an external force is applied to each wafer strip 1 to divide it along the second streets 3, thus obtaining the individual devices. FIG. 4A is a plan view for illustrating a conventional dividing method in which a laser beam is applied from one end to the other end of each second street 3 of the wafer strip 1, and FIG. 4B is a cross section taken along the line A-A in FIG. 4A. As shown in FIGS. 4A and 4B, the modified layer 5 is formed along each second street 3 by applying the laser beam from one end 1b to the other end 1b of the wafer strip 1 along each second street 3. In this conventional dividing method, however, there is a possibility that the modified layer 5 inside the wafer strip 1 may be partially removed at the opposite ends 1b by ablation. As a result, debris 6 may be generated from the opposite ends 1b as shown in FIG. 4B, causing a degradation in quality of a device to be obtained.
This problem may be solved by another conventional dividing method shown in FIGS. 5A, 5B, and 5C. FIG. 5A is a plan view for illustrating such another conventional dividing method in which a laser beam is applied along each second street 3 of the wafer strip 1 except the opposite ends 1b of each second street 3, FIG. 5B is a cross section taken along the line B-B in FIG. 5A, and FIG. 5C is a plan view of a device 8 obtained by applying an external force to the wafer strip 1 shown in FIG. 5A. In this conventional dividing method, however, it is difficult to control the laser beam so as to stop the application of the laser beam just before the opposite ends 1b of the wafer strip 1. Further, as shown in FIG. 5B, each second street 3 of the wafer strip 1 has a pair of areas 7 where the modified layer 5 is not formed near the opposite ends 1b. In this case, there is a possibility that the device 8 obtained by applying an external force to the wafer strip 1 shown in FIGS. 5A and 5B may have chipping 9 such as burrs and cuts as shown in FIG. 5C.